I can't speak for the entire "Is it legal" question, but I DO know that at least some commercial radio licenses specify a maximum number of mobile and base radios they are licensed for. I know the one for my employer only has one base and 40 mobiles on their FCC license. You might want to check and see how many units their license covers before you assume that there are "available" / unused license slots.

My employer is using FAR below their number of units, so I know I'm covered there.

I can't speak for the entire "Is it legal" question, but I DO know that at least some commercial radio licenses specify a maximum number of mobile and base radios they are licensed for. I know the one for my employer only has one base and 40 mobiles on their FCC license. You might want to check and see how many units their license covers before you assume that there are "available" / unused license slots.

My employer is using FAR below their number of units, so I know I'm covered there.